NASA's latest planet hunter, the Transiting Exoplanet Survey Satellite ( TESS ), has discovered three new worlds – one slightly larger than Earth and two of another type found in our solar system – orbiting a nearby star. The planets span an observed gap the size of known planets and are considered one of the strangest targets for future study.
TESS Object of Interest (TOI) 270 is a faint, cool star that is more commonly identified by its catalog name: UCAC4 191-004642. The M dwarf star is about 40% smaller in size and mass than the sun and has a surface temperature that is about one third lower than that of the sun. The planetary system is located in the southern constellation of Pictor about 73 light-years away.
"It is this system developed by TESS to find small, temperate planets that pass or pass in front of an inactive host star. One of them does not exhibit excessive stellar activity such as torches, "said lead researcher Maximilian Günther, a postdoctoral fellow from Torres at the Kavli Institute of Astrophysics and Space Research at the Massachusetts Institute of Technology ( MIT ) at Cambridge. "This star is quiet and very close to us and therefore much brighter than the host stars of comparable systems. With extended observations, we can soon determine the composition of these worlds, determine if there are any atmospheres, what gases they contain, and more.
An article describing the system was published in the journal Nature Astronomy and is now available online.
The innermost planet, TOI 270 b, is probably a rocky world about 25% larger than Earth. It orbits the star every 3.4 days at a distance about 13 times closer than Mercury's orbit around the sun. Based on statistical studies of known exoplanets of similar size, the science team estimates that TOI 270 b has a 1.9 times larger mass than Earth.
Because of its proximity to the star, Planet b is an oven-hot world. Its equilibrium temperature, ie the temperature based solely on the energy it receives from the star and ignoring the additional warming effects of a possible atmosphere, is around 254 degrees Celsius.
The two other planets, TOI 270 c and d, are 2.4 and 2.1 times larger than Earth, respectively, and orbiting the star every 5.7 and 11.4 days. Although both are only about half as large, they may resemble Neptune in our solar system, with the composition being dominated by gases rather than rocks, and they probably weigh 7 to 5 times the mass of the Earth.
All planets are expected to be tethered tethered to the star, meaning they only spin once in each orbit and always point to the same side as the moon in its orbit around the earth.
Planet c and d are best described as mini-neptune, a kind of planet not seen in our own solar system. The researchers hope that further exploration of TOI 270 can help explain how two of these mini-neptunes have formed next to a nearly Earth-sized world compact, that the orbits of Jupiter and its moons are the closest reasonable comparison in our own solar system offer, as shown here. Credits: NASA's Goddard Space Flight Center
"An interesting aspect of this system is that its planets span a well-established gap in known planetary sizes," said co-author Fran Pozuelos, a postdoctoral fellow at the University of Liège in Belgium. "It is unusual for planets to have a size between 1.5 and 2 times the Earth for reasons that are probably related to the formation of planets, but this is still a highly controversial topic. The TOI 270 is an excellent laboratory to study the edges of this gap and to better understand how planetary systems arise and evolve. "
Günther's team is particularly interested in the outermost planet, the TOI 270 d. The team estimates the planet's equilibrium temperature at about 150 degrees Fahrenheit (66 degrees C). This makes it the most temperate world in the system – and as such a rarity among known transit planets.
"TOI 270 is perfectly positioned in the sky to study the atmosphere of its outer planets with NASA's upcoming James Webb Space Telescope. Said co-author Adina Feinstein, a doctoral student at the University of Chicago. "Webb will be able to watch them for over half a year, which could make for really interesting comparative studies between the atmospheres of TOI 270 c and d."
The team hopes that further research can identify more planets beyond the three now known. If the planet d has a rocky core covered by a dense atmosphere, its surface would be too warm for liquid water, which is considered a key requirement for a potentially habitable world. However, follow-up may reveal more rocky planets a little further away from the star where liquid water can accumulate on their surfaces at lower temperatures.
TESS is a NASA Astrophysics Explorer mission, managed and operated by MIT in Cambridge, Massachusetts, and managed by NASA's Goddard Space Flight Center. Other partners include Northrop Grumman of Falls Church, Virginia; NASA's Ames Research Center in Silicon Valley, California; the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts; The Lincoln Laboratory of MIT; and the Space Telescope Science Institute in Baltimore. The mission involves more than a dozen universities, research institutes and observatories worldwide.